Improved Calculation Method of Soil Bearing Capacity of Jack-Up Rig Based on Field Simulation Experiment

Abstract

Abstract The spudcan penetration depth is directly determined by the soil bearing capacity which is the basis of punch-through analysis. Through the analysis of the SNAME method, it is found that it has great limitations: without considering lateral friction, preloading speed, soil backfill effect and group effect of spudcans. The predictive accuracy of soil bearing capacity increases by 40% through the improved calculation method based on field simulation experiment with the above four factors into consideration. The field experiment was conducted in the Bohai shallow sea to simulate actual penetrating process. Firstly, there were 9 sampling positions and 10 soil samples per meter were obtained from each position. Secondly, the soil pressure sensors were inlaid into the spudcans at the bottom and side. Thirdly, the bearing capacity was studied by changing the preloading speed of 2t/h, 4t/h, 6t/h, 8t/h and 10t/h. Fourthly, the bearing capacity was researched by changing the soil backfill ratio of 0%, 10%, 20%, 30% and 40%. Fifthly, the group effect was observed when the three spudcans were ballasted one by one. The vertical soil stress is obtained by the bottom sensors and the vertical soil bearing capacity is calculated. The radial soil stress is gained by the side sensors and the lateral friction is calculated. The field simulation experiment shows that the lateral friction makes up about 3% of the total bearing capacity. The larger the preloading speed is, the smaller the bearing capacity is. In addition, the excessive preloading speed may lead to punch-through. The larger the soil backfill ratio is, the smaller the bearing capacity is. The group effect is obvious when the three spudcans are ballasted one by one and the penetration depth of the third spudcan is the shallowest normally. This improved method has been effectively applied to EP oil field for HYSY941 and DF HTHP gas field for HYSY944. The EP and DF are typical layered soil in South China Sea. The case studies were carried out to verify the method, the results showed that the maximum relative errors of spudcan penetration depth were within 5% and the method successfully avoided several punch-through accidents. The novelty of this paper is to introduce an improved method based on field simulation experiment. The predictive accuracy of spudcan penetration depth is higher and the punch-through accidents can be successfully avoided by the proposed method considering lateral friction, preloading speed, soil backfill effect and group effect of spudcans. The improved method is consistent with the actual engineering and satisfies the real need of field operations.